Measurements

I get questions from people who build
experimental (or not experimental) solar devices, and would like to measure
their performance. Measuring performance can be very helpful to see
if the device is working well, and/or to measure the benefits of design changes.

So, here are some simple
ways that I use to measure stuff. Since most of us do not have the test
budget of a GE or Boeing, the emphasis is on cheap ways to get good data.

There are probably better, easier, and
cheaper ways to do all of this, so if you know of any lets hear about them!
Gary.

Since most of my work is in the solar
thermal area, there is not much (nothing in fact) on measurements for PV
systems, wind generators, or the like. If you have some experience in
these areas and would like to pass along some tips, techniques, or advice --
please do.

This
very simple and inexpensive Dwyer gage measures air velocities in the range
from 10 fpm up to 400 fpm. The incoming air stream deflects a vane up
the meter scale. It has just about the right amount of damping to give
steady readings, but still react to changes. I use this meter and like
it a lot. Note that the upper limit of 400fpm is to low for typical
flow in fan forced system. It is ideal for the flow velocities
encountered in thermosyphon or natural convection systems. About
the only negative is that it can only measure horizontal flow, but you can
change the flow direction with something like a gutter pipe elbow so
it is horizontal, and measure this. Only $25.

Other suppliers make similar meters -- a search under "Wind Meters" will
turn some up.

These
wind meters use a small turbine whose rotation rate is measured to determine
air velocity. This Kestrel works well from about 70 fpm up to
several thousand fpm.

I have used the one in the picture for several years and think
it works quite well. There are several brands and many models. This model is
about $80. They also come in models that include other sensors for
temperature, humidity, altitude, ... I think that some now even offer
some form of logging.

This is a great and practical piece on how to measure airflows accurately
using timed filling of garbage bags, or simple pressure difference across a
flow resistance, where the pressure difference is generated with a simple
laundry basket.

When you buy the thermometers, try to select ones from the bin that read the same
temperature. In most cases, you will be interested in the difference
between the readings on two thermometers, so the absolute accuracy is not
usually critical, but having thermometers that are all off by the same
amount helps.

When you place them, try to get the bulb into the air stream you are
measuring, and keep them out of direct sun (this may require a shade of some
type).

For most projects, you will get better results if you take and record
several readings over a time span. If more accurate measurements
are needed, the thermometers can be calibrated as described below.

Every
once in a while, bring them all together and see if they read the same
temperature -- throw away any that don't.

If you want to move up to a more accurate and precise thermometer, here is one example.

This is the Hanna Checktemp. It is thermistor based with a stainless proble that can be inserted in air or liquid. It maintains an accuracy of +- 0.5F over its -58F to +302F range. It comes with a calibration certificate and it has a built in calibration test you can run any time.

About $25.

Thermistors

Thermistors are semiconducting
devices whose resistance varies as a function of temperature. This
resistance change is used to measure temperature. They are
relatively cheap, can be quite accurate (e.g. 1/4F over the solar thermal
range), and are fairly rugged when packaged well.

Picture of logger with
thermistor above, and
thermistor packaged in stainless can.

They require some circuitry and a display to show the temperature, or a
data logger to record the temperature readings. Or, an ohm meter can
be used to find the resistance, and then look up the temperature in a table.

If you decide to invest in a data logger, the loggers that use thermistor sensors
tend to be among the most affordable.

The picture shows a thermistor sensor
enclosed in a stainless steel can, and connected to a cable that is plugged
into a data logger.

Thermocouples

A thermocouple is a junction of two
different metals. Temperature changes at the junction generate a
voltage that is proportional to the temperature.

IR thermometers measure surface temperature by measuring the IR radiation emitted
from the surface. About $50.

You can use them for measuring surface
temperatures in solar thermal experiments, and also to see if your tires are
overheating, or your beef stew is up to temperature -- no household should be
without one.

They are often the easiest way to measure surface temperatures.
Some caution must be used, in that they assume that the IR emissivity of the
the surface is 0.95. Many surfaces are close to this, but some
surfaces (e.g. polished metals) are much less. In some cases this
ambiguity can be removed by putting a piece of dark colored tape on the
surface, and doing the measurement on the tape. You can get air
temperature by waving a sheet of paper around in the air, and then measuring
the paper surface temperature with the IR thermometer.

I have a little
matchbook size one that I carry around some of the time -- they are great
for getting a better understanding of the thermal physics of the world
around us. The matchbook size model costs $25..

Thermal Imaging Cameras

Thermal
imaging cameras offer an unmatched capability to view the temperatures over
a whole surface.

They have been almost prohibitively expense, but over the last few years
have come down a lot as some entry level lines have been introduced with
lower prices that have been steadily dropping. The lowest priced ones
are now a bit over $1000 -- still a lot, but much better than $10,000.

This is an IR (thermal) camera that you can build for a couple hundred(?) dollars.

Instead of the array of sensors that most thermal imagers use, it uses a single IR sensor that is scanned over the target object, then an image is put together from the set of readings taken during the scan.

The upside is the price and the downside is the time it takes to scan an image (about 6 minutes?). Pretty clever.

Water Flow
Measurements

Timed Filling of
A Bucket

Gary

If you can arrange it so that the
fluid flow to be measured can be diverted to a container of know size you can get very
accurate flow rate measurements by just timing how long it takes to fill the
container. A stopwatch is helpful for accurate
measurements. Make sure that whatever you do to divert the
flow does not itself change the flow rate. If you do this carefully, it will provide better accuracy than
all but the most sophisticated flow meters, and do it for zero cost.

In
the picture, there is a submersible pump in the bucket that pumps water
around the collector, and back into the bucket via the line with the flow
control clamp on it. To measure flow, the line going back into the
bucket is redirected to the 2 qt measuring bowl in the background, and the
fill time measured.

A
sampling of commercially available flow meters at the low end of the price
scale.

Pressure
Measurements

Water Manometer

A water manometer can be used to
measure pressure differences in inches of water. A manometer can be easily
made from transparent plastic tubing bent into a U shape. The
height of the U should b a bit greater than the maximum pressure
differential you will want to measure. To use
it:

Fill the U tube up about half way full with water.

Connect the top of one side of the U to the pressure source to be measure
using a tube. Leave the other end of the U open.

With the pressure applied, measure the difference in height between the water columns in the two
arms of the U. This is the pressure in inches of water. One
inch of water is 0.036 psi.

Ordinary ink pen ink can be used to dye the water. Adding a little
bit of detergent prevents breaking up of the water column.

Two
water manometers used to measure the top dome and bottom dome inflation
pressures of an inflatable heliostat. The difference in pressure is
used to focus a flexible aluminized Mylar mirror. You can see that the
pressures are nearly equal, and about 6 inches of water.

These
Dwyer Magnehelic gages provide an accurate and relatively cheap way to
measure small pressure differences (as commonly occur in solar air systems).
About $50, but often available on ebay for much less. Available with
full scale readings from 0.25 up to more than 100 inches of water.
They are for measuring gas pressures only (not water).

Power
Measurements

Kill-A-Watt

This
amazing gadget measures voltage, current and power (in watts) consumed
by a 120VAC appliance plugged into it. It will also accumulate
energy used (KWH) over the period of time the appliance is plugged into it.

The claimed accuracy is 0.2, which is amazing. If you bought the equivalent of this as a "lab instrument", it would
probably cost a thousand dollars.

If
you want to know how much power a device is using, and the device has a
fixed and known power consumption when its on, then you can hook up an
electric clock across the connection that goes hot when the device powers
on. The clock will then only run when the device is powered up.
Just set the clock for midnight at the start, and note what time its
pointing to 24 hours later, and you have the daily on time.

The picture shows using an electric timer to record how many hours
my water heater runs each day by powering the timer from the fan that runs
whenever the burner flue exhaust fan is on (which runs only when the burner
is on).

The
link describes a way to use the utility company watt-hour meter on your
house to estimate the power consumed by an electrical load of interest.
This is based on each turn of the meter disk representing a fixed number of
watt-hours -- often 3 watt-hours per turn.

The 2nd link tells you how to
read the totals on your electric and gas meters.

This
easy to install power meter tells you how much electricity your house is
using (and has used).
The idea being that getting instant feedback when you turn on lights,
appliances, ... will help you cut down on electricity use.
Kind of like driving a Prius and seeing what your current mpg is all the
time.

I
finally bit the bullet and got an Apogee Pyranometer in order to be able to
measure sun intensity accurately. The Apogee outputs 2 mv per watt/m^2
of solar radiation (so about 2 volts for full sun). It is factory
calibrated to +- 5%, and is cosine corrected. About $180. It
works well with the Onset logger which will convert the voltage readings
directly to watt/m^2. So far, I have been quite happy with it, and
have a number of projects planed for it.
Shown here set up to determine if the corrugations on SunTuf glazing reduce
its transmission
Details ...

This
compact and self contained logger measures light intensity in lumens/ft^2
and temperature. It will handle full sunlight, and you could
correlate lumens/ft^2 to watts/m^2 to get a rough idea of solar intensity.
It has an internal battery, and requires no connection to computer during
logging. Case is a bit like a small pill bottle, and is water
resistant. Price is about $40, but you also need the coupler and
software, which add many dollars -- so this option might be best if you have
or plan to use other Onset loggers.

The
small logger in the picture (white box) logs the on time for the motor its
attached to. No wiring connections -- the logger senses the motor
magnetic field.

This Onset motor on/off monitor is the one I use -- I'm sure there are
others. It is very easy to use -- just tape it to the side of the
motor.
Price of this one is $79 + software.Lots of uses -- some examples ...

Inexpensive digital versions of "fish" or "game" scales are now available.
The one I have is reliable and accurate.

Gravity and
pulleys

The weighted bucket applies a known force to the edge of the heliostat
via the rope over a pulley.
Not very elegant, but it works!
Used to estimate the heliostat beam displacement for high wind conditions.
I have found the digital bathroom scales to be an accurate way to measure
the weights, but known quantities of water at 8.3 lb per gallon can also be
used.

Data LoggersData loggers allow you to record temperatures, voltages, events, ... at
regular intervals.
The interval can typically range from once a second or less to hours, and
the time period over which data is logged can be minutes to months.
Values recorded by the logger are typically transferred to a computer for
processing or plotting.
Data loggers are typically NOT required for most solar thermal experiments
-- you can get equally good results by just manually reading and recording
sensor values -- the loggers just make life easier.

A
home made temperature data logger based on the inexpensive Dallas
Semiconductor DS18B20. This logger supports up to 64 sensors.
The sensors use the Dallas Semi "one wire" bus, which means that the sensors
can be a 100 ft or more from the logger, and require minimal cabling, and no
separate power. The logger interfaces the sensors to a Windows PC.
More than 100 of these logger have been built.

There are other versions of
loggers that make use of the "One Wire" sensors. Try a google for the
latest.

Good
Home Power article on using an Onset Computer logger to measure power
consumption for PV system sizing.
This is a year 2000 article, so some of the hardware may be a bit out of
date, but the techniques are still applicable.

The system logs many sensors and provides real-time and historic data
over the internet.

Phil
describes his system for monitoring the status and performance of his
heating system in "Our
Cool House". This system uses the 1-Wire bus (and more) to allow
several types of sensors to be logged. A dedicated micro- processor is
used to reduce the power consumption of the system. The logged values
are sent to a web server, and can be accessed from by web browser.
Phil sells kits for the logger main board, and for some of the sensors.
He also provides the software, including source code (so you can customize
if you wish).Our Cool
House is Phil and Lisa's very interesting, passive solar, earth
sheltered, geothermal house -- their website provides lots of good
information on the design an building of their house. Its in our "I
Did It" homes section.

The
myDTC is primarily a controller (a very flexible one), but also includes a
logging function that is able to store a number of sensor values over a
longish period of time.

One of the sensors offered is a solar radiation sensor, which can also be
logged.

Some Sample Measurement SetupsHere are a few sample test setups that I had some pictures for.
This is not rocket science -- one of the nice things about Solar Thermal is
that you can do good work with simple test setups. Here are some
simple examples that might be helpful to folks who have not done much of
this stuff.

A Test Solar
Water Collector

A
simple setup that measures the output of a small collector built to test a
new design.

Some
simple techniques to measure collector performance, and determine if design
changes that you make actually improve performance.
A very rough way to estimate collector efficiency.
A list of common collector design misconceptions. More...